QB_PH_SEC9_012: Particle Physics & Fundamental Interactions

What is the quark content of the positively charged pion $\pi^+$?

Correct Answer

### Step-by-Step Derivation & Verification: 1. **Meson Composition**: Mesons consist of a valence quark-antiquark pair ($q_1 \bar{q}_2$). 2. **Quantum Numbers of $\pi^+$**: - Charge: $Q = +1$ - Baryon number: $B = \frac{1}{3} - \frac{1}{3} = 0$ - Strangeness: $S = 0$ - Isospin: $I = 1, \; I_3 = +1$ 3. **Quark Assignment**: - $u$ quark has $Q_u = +2/3, \; I_{3u} = +1/2$ - $\bar{d}$ antiquark has $Q_{\bar{d}} = -(-1/3) = +1/3, \; I_{3\bar{d}} = -(-1/2) = +1/2$ - Net charge: $+2/3 + 1/3 = +1$ - Net $I_3$: $+1/2 + 1/2 = +1$ 4. **Conclusion**: $\pi^+ = u\bar{d}$. Option (A) is correct.